This invention is directed to a method of hydraulically fracturing a subterranean earth formation penetrated by a well and more particularly to using in a hydraulic fracturing technique an aqueous fracturing fluid comprised of a complex formed by the reaction of a polysaccharide and a galactomannan.
Hydraulic fracturing techniques have been widely used for stimulating wells penetrating subterranean hydrocarbon-bearing formations by creating fractures which extend from the wells into the formation. These techniques normally involve injecting a fracturing fluid down a well and into contact with the subterranean formation to be fractured. A sufficiently high pressure is applied to the fracturing fluid to initiate a fracture in the formation and the fracturing fluid is injected down the well at a sufficiently high rate to propagate the fracture thereinto. Propping materials are normally entrained in the fracturing fluid and are deposited in the fracture to maintain the fracture open.
Many different types of fracturing fluids have been employed. The evaluation of hydraulic properties of fracturing fluids is reviewed in an article, "What to Learn about Hydraulic Fracturing Fluids", T. C. Buchley and D. L. Lord, The Oil and Gas Journal, Sept. 17, 1973, pp. 84-88. Buchley et al. state that present-day fracturing fluids are classified as Newtonian, polymer solutions, crosslinked polymer solutions, emulsions, micellar solutions, and gelledorganic liquids in solution with a liquefied gas. They teach that a fracturing fluid should be compatible with both rock-matrix material and natural fluids contained within the matrix pores. Newtonian fluids are used infrequently as fracturing fluids because they produce large wellbore pressure drops. They are significant, however, because they are the main ingredients in the formulation of fracturing fluids. It is the addition of polymers and/or other components to water, oil, or alcohol which produces most present-day fracturing fluids.
With respect to crosslinked polymer solutions, Buchley et al. indicate that these have received more attention than other fracturing fluids in the past several years. In field preparation of crosslinked polymer solutions a water-base gel is first prepared by adding natural or synthetic polymer to the base fluid at a concentration of 40 to 80 lb/1000 gal. Next, an appropriate chemical is added to the thickened water-base gel to crosslink the polymer molecules. In field application the crosslinking agent is injected continuously at the surface as the gelled fluid is pumped downhole.
In a paper entitled "New Generation of Frac Fluids" by J. L. White and R. B. Bosene, presented at the 24th Annual Technical Meeting of the Petroleum Society of CIM in Edmonton, May 8-12, 1973, there is described new, very viscous frac fluids and low-damage fluids to improve well productivity. These fluids exhibit low friction pressure down tubular goods and high viscosity in the fracture and are low damaging and exhibit low fluid loss and positive gel breakdown. According to White et al. there are three basic materials used to prepare water-base thickened fluids: guar gum, cellulose derivatives, and a synthetic polymer. The high viscosity of these fluids is achieved either by crosslinking or using high concentrations of polymers that impart good viscosity and friction reducing properties. White et al. further teach that there are four viscous water-base fluids available which cover a wide temperature range. The four new fluids are a crosslinked guar gum, a crosslinked cellulose derivative, and two fluids using a high concentration of synthetic polymer. The crosslinked guar fluids develop higher viscosities than the other fluids but this high viscosity drops sharply at temperatures above 220.degree. F.
White et al., in discussing the rheology of the thick frac fluids, point out that in the case of non-Newtonian fluids, the shear stress is not proportional to the shear rate and that the term "apparent viscosity" is applied to these fluids. This is the viscosity the fluid has at a specific shear rate. The apparent viscosity is the ratio of shear stress to shear rate. The apparent viscosities at fracture shear rates for the gels described in the paper were experimentally determined using a high temperature, high pressure Fann Viscosimeter, Model 50.
It is generally recognized that the term "non-Newtonian" fluid generally implies that the fluid is nonlinear in the sense that the viscous resistance of the fluid is a function of shear rate versus shear stress.
In the paper entitled "Polysaccharide Derivatives Provide High Viscosity and Low Friction at Low Surface Fluid Temperatures", by J. L. White and J. O. Means, Journal of Petroleum Technology, September 1975, pp. 1067-1073, reference is made to low-residue polysaccharide derivatives that achieve high viscosity and low friction levels much more rapidly than conventional thickeners.
In U.S. Pat. No. 3,710,865 there is described a fracturing method wherein an oil-in-water emulsion is used as a fracturing fluid. The emulsion contains a major volume proportion of an internal liquid hydrocarbon phase. The water phase comprises an aqueous polymer solution having a viscosity of at least 10 centipoises and preferably between 10 and about 100 centipoises at 70.degree. F. and a shear rate of 511 reciprocal seconds. The aqueous polymer solution can be prepared by adding water-soluble polymeric thickening agent to the water prior to mixing the water and oil phases together. The polymeric thickening agent can be any one of a variety of long-chain, water-soluble polymers capable of building the viscosity of an aqueous solution. These polymers are commonly referred to as gums. Synthetic and modified polymers and natural gums can be used. Natural gums include guar gum, gum arabic, gum tragacanth, gum karaya, and the like. Also usable are the microbial fermentation gums such as dextran and the heteropolysaccharides produced by the bacteria of the genus Xanthomonas. The synthetic and modified polymers include the acrylic polymers such as polyacrylamide and polyacrylic acid; the vinyl polymers such as polyvinylpyrrolidone and polyvinylcarboxylic acid neutralized with a long-chain amine and a common base; and the cellulose derivatives such as sodium carboxymethylcellulose, sodium carboxymethylhydroxyethylcellulose, methylcellulose, hydroxyethylcellulose, and ethylhydroxyethylcellulose. In some applications it may be desirable to use both natural and synthetic polymers since their respective chemical effects on the emulsion may be different.
In U.S. Pat. No. 3,760,881, there is described a method of treating a subterranean formation surrounding a wellbore by injecting into the formation a water-based viscous fluid containing a complex produced by the reaction of an aliphatic quaternary ammonium compound with a water-soluble compound selected from the group consisting of monosaccharides, disaccharides, trisaccharides, polysaccharides, and long-chain synthetic hydroxylated polymers which yield such complexes at a temperature between about 20.degree. C. and about 205.degree. C. or higher. Complexes prepared by the reaction of long-chain quaternary ammonium halides with high molecular water-soluble polysaccharides such as guar gum or a similar galactomannan are particularly effective for purposes of the invention. Water-soluble polysaccharides of vegetable, animal or microbial origin, including both the structural and nutrient types are suitable for use. Among the examples given of suitable water-soluble polysaccharides are galactomannans, natural gums such as gum karaya, gum tragacanth, and the like, and polysaccharides produced by bacteria of the genus Xanthomonas.
In U.S. Pat. No. 3,483,121 there is described an aqueous fracturing solution that contains a hydroxyalkyl ether of a galactomannan gum having a degree of substitution of 0.1-5.0. The hydroxyalkyl galactomannan gums are made by reacting galactomannan gums such as guar gum and locust bean gum with an alkylene oxide having at least two and preferably three or more carbon atoms.
In U.S. Pat. No. 3,768,566 there is described a fracturing fluid comprising an aqueous fluid having a pH of less than 7, a water-soluble alcohol, and a crosslinked polysaccharide. A method is described whereby the viscosity of a fluid is increased at a time when the fluid is being subjected to temperature which tends to reduce the initial viscosity of the fluid. The viscosity is increased by the hydration of an additive which is a polysaccharide that has been crosslinked such that the polysaccharide's hydration rate is greatly retarded at temperatures below about 100.degree. F. but which may be hydrated at temperatures above 140.degree. F. The viscosity increasing additive is a hydratable polysaccharide crosslinked with a compound selected from the group consisting of dialdehydes having the general formula EQU OHC(CH.sub.2).sub.n CHO,
wherein
n is an integer within the range of 0 to about 3.
Polysaccharides useful for forming the retarded gelling agent are hydratable polysaccharides having a molecular weight of at least about 100,000. Suitable hydratable polysaccharides are hydratable galactomannan gums, hydratable glucomannan gums and hydratable cellulose derivatives. Examples of suitable hydratable polysaccharides are guar gum, locust bean gum, karaya gum, carboxymethylcellulose, carboxymethylhydroxyethylcellulose and hydroxyethylcellulose.
In an article entitled "Shapely Polysaccharides" by D. A. Rees, Biochemical Journal, Vol. 126, 1972, pp. 257-273, it is said that the Xanthomonas polysaccharide does not form gels by itself, but, when mixed with locust bean galactomannan, which also does not gel alone, a stiff rubber gel can be formed.
In an article, entitled "Useful Incompatibility of Xanthan Gum with Galactomannans", Peter Kovacs, Food Technology, Vol. 27, No. 3, March 1973, pp. 26-30, it is said that one of the most useful incompatibilities is the reaction of xanthan gum with galactomannans, such as locust bean gum and guar gum. When xanthan gum is combined with guar gum, a synergistic increase in viscosity occurs; in most cases this increase is not dramatic. However, when xanthan gum is combined with locust bean gum, a highly significant viscosity increase occurs at low concentrations, and as the colloid concentration is increased, a thermoreversible and highly cohesive gel is obtained. It is further said that maximum gel strengths are obtained with xanthan gum: locust bean gum ratios in the range of 6:4 to 4:6.